1. Field of the Invention
The present invention relates to an improved method of manufacturing explosive articles comprising detonating cord, sheet or shaped articles and, in particular, to a method for incorporating explosive material into such articles.
2. Related Art
Detonating cord is typically manufactured by one of three processes. In the wet process, particulate core material (i.e., pentaerythritol tetranitrate, "PETN") is mixed with water and other chemicals to provide a non-cap-sensitive slurry which is formed into a cylindrical rod about which a braiding machine braids yam or the like to form a cord of explosive having a cylindrical braided fabric casing around it. The slurry within the braided cord is then dried and, after the drying step, the braided cord is coated with plastic to provide additional strength and protection. The speed of a wet process manufacturing facility is limited by the length of time necessary to dry the slurry within the braided cord prior to application of the plastic coating. In the dry process, dry particulate core material (i.e., particulate PETN) is gravity-fed from a hopper into a plastic or paper tube as the tube is being formed from plastic or paper tape, over which a fabric sheath is helically spun. Dry process manufacturing requires frequent reloading of the PETN supply hopper with the explosive to provide a uniform core loading during the manufacturing process. In both the wet process and the dry process, the finished product comprises a core of explosive material surrounded by ajacket. In the extrusion process, which is disclosed in U.S. Pat. No. 4,232,606 issued on Nov. 11, 1980 and entitled "Explosive Connecting Cord", a billet of PETN is mixed with a plastic binding agent in order to provide an extrudable plastic-bonded PETN material having a viscosity comparable to that of modeling clay. The plastic-bonded PETN material is then extruded into a cord and a plastic jacket is applied over the extruded strand of PETN material. Optionally, strands of reinforcing yarn may be applied to the extruded PETN material and the plastic jacket applied thereover.
The publication Explosion and Explosives, is published by the Government Chemical Industrial Research Institute, Hirastsuka, Kanagawa, Japan. In Vol. 32, No. 6, 1971, at pages 27-35 of the published English language version, there appears an article by Senzo Oinuma, Masayoshi Kikkawa and Shohachiro Okubo entitled "A Thin Lead Azide Detonating Fuse Of Thread Type". This article (the "Oinuma et al article") discloses the fabrication of a lead azide detonating fuse by reacting lead nitrate and sodium azide within the weave of the fabric (thread) comprising the body of the fuse. The fuse is fabricated by boiling a cotton or linen thread in water to drive out any air, then immersing it in an aqueous solution of concentrated lead nitrate. The impregnated thread is then immersed in ethanol to precipitate the lead nitrate largely within the fibers of the thread, with some surface crystals being formed. The latter are shaken off the surface of the thread. Finally, the thread is immersed in a sodium azide solution to react the sodium azide with the lead nitrate within the thread for 1 to 2 hours to form small lead azide crystals within the weave of the thread. The formation of large lead azide crystals on the surface of the fabric is said to be prevented by shaking the lead nitrate crystals off the surface of the thread as described above, before the thread is immersed in the sodium azide solution. The finished product may be wrapped in cellophane tape to prevent spilling of the lead azide during handling.
The publication Journal of the Industrial Explosives Society, Japan contains, in Volume 35, No. 5, 1974, at pages 220-226, an article by S. Oinuma entitled "Lead Azide-Containing Sheet Initiator". This article ("the Oinuma article") discloses a method of preparing lead azide sheet-like material by treating a molded resinous sheet containing lead acetate with an aqueous solution of sodium azide. The raw material sheet is obtained by molding lead acetate powder with a binder and osmosis-promoters in an acetone solution. The acetone is evaporated from the solution in a pan to provide the raw sheet material. The authors point out that the technique of impregnating fibers with a lead nitrate solution (as discussed in the above-noted Oinuma et al article in Explosion and Explosives) does not work with the resinous sheet material because the sodium azide does not penetrate through the resinous sheet material. Consequently, lead acetate powder is molded within the sheet as described above. The raw material sheet is soaked in an aqueous solution of sodium azide, washed with water and dried. A concentrated rather than a dilute aqueous solution of sodium is used for the impregnation of the raw material sheet because the dilute solutions tend to form the lead azide on the surface of the sheet rather than within it.
Japanese Patent Document J 57-188 491 describes the manufacture of a pyrotechnic fuse by the impregnation of synthetic (nylon, polyester, acrylic, etc.) or natural (cotton, jute, etc.) fiber yarn with a pyrotechnic paste composition comprising a solvent mixed with nitrocellulose, a flame colorant and a fireworks powder.
U.S. Pat. No. 5,518,807 of Chan et al discloses depositing upon a porous substrate of oxidizing polymeric film, such as polytetrafluoroethylene ("PTFE"), an oxidizable material such as a metal or alloy selected from one or more of lithium, sodium, magnesium, beryllium, etc. The polymer, which is porous so that the metal may be deposited within the pores to promote intimate admixture and rapid reaction, and the metal conjointly react to exothermically generate energy for use, e.g., in inflating an air bag. (Column 1, lines 40-46.) In one embodiment, the deposited metal reactant is supplemented with an incendiary material such as sodium azide, which is also deposited within the pores of the polymer. See column 1, line 66, through column 2, line 4. In Example 2 (column 4), the addition of sodium azide is exemplified by soaking the porous PTFE film in a saturated aqueous solution of sodium azide and drying it to fill 50% of the available 90% pore space with solid sodium azide, leaving 40% of the pore space available to receive the metal. The result of addition of the incendiary material is said to be enhanced reaction speed and energy output.